The oxidation of .alpha.,.beta.-unsaturated aldehydes to the corresponding .alpha.,.beta.-unsaturated carboxylic acids or carboxylic acid salts is well known in the art. Improved yields of the desired acid are known to be obtainable by oxidation of the unsaturated aldehyde with an oxidizing gas in an aqueous alkaline solution in the presence of a silver catalyst. U.S. Pat. Nos. 2,930,801; 3,162,682 and 3,839,437 disclose methacrylic acid production from methacrolein oxidation in a strongly alkaline medium catalyzed with silver as illustrated by the following: ##STR1## Active life of the silver catalyst is disclosed by U.S. Pat. No. 2,887,496 to be prolonged by conducting such oxidation in the presence of a mixture of triethanolamine and tetrasodium salt of ethylenediaminetetraacetic acid.
Oxidation catalysts having copper (II) oxide alone or in combination with other metals and metal oxides, including silver, are also known. Polyacroleins may be oxidized to corresponding polyacrylic acids as described in U.S. Pat. Nos. 3,387,029 and 3,546,286. Polyacrolein and polymethacrolein oxidation is disclosed to be conducted in an aqueous alkaline solution maintained by an alkali hydroxide or its corresponding carbonate or bicarbonate.
When methacrolein has been oxidized by the prior art alkaline solution silver catalyzed processes, production of the corresponding acid salt is accompanied by the formation of methallyl alcohol and isobutyrate salts as by-products. Formation of those by-products represents a diminution of yield of the desired methacrylate salt and further causes problems in purification of methacrylic acid formed upon acidification of the salt-form product.